1. Field of the Invention
The present invention relates to a transformer, especially to a symmetric planar transformer having adjustable leakage inductance.
2. Description of the Prior Art(s)
A transformer is a device that transfers electrical energy from one circuit to another circuit through inductively coupled conductors (a primary winding, a secondary winding and a magnetic core of the transformer). Imperfect coupled primary and secondary windings of a conventional transformer have a coupling coefficient less than 1 so that a leakage inductance occurs. Since the leakage inductance has an influence on power conversion efficiency of the transformer, designers of the conventional transformer endeavor to increase coupling efficiency of the primary and secondary windings of the conventional transformer to reduce the leakage inductance and power loss upon voltage regulation.
Instead of reducing the leakage inductance, a recent development of power supply system in an electronic product actively makes use of the unavoidable leakage inductance. For example, the leakage inductance (L) and a capacitor (C) compose a LC resonant circuit. A soft switch using the LC resonant circuit has reduced damage possibility, minimized noise and improved performance.
With reference to FIGS. 16 and 17, a conventional transformer having adjustable leakage inductance (U.S. Pat. No. 7,236,077) comprises a bobbin 83, a primary winding 81, a secondary winding 82 and a magnetic core assembly 84. The bobbin 83 has a winding section 831 formed on the bobbin 83, and a channel 832 formed through the bobbin 83. The primary winding 81 and the secondary winding 82 are wound around the winding section 831 of the bobbin 83, insulate from each other and are overlapped partially with each other. Part of the magnetic core assembly 84 is mounted into the channel 832 of the bobbin 83. Thus, an overlap region is defined at where the primary winding 81 and the secondary winding 82 are overlapped with each other, and a non-overlap region is defined at where the primary winding 81 and the secondary winding 82 are not overlapped with each other. A leakage inductance of the transformer is adjusted according to an overlap ratio of the overlap region to the sum of the overlap region and the non-overlap region.
In general, the secondary winding is center-tapped and has a connecting terminal protruding from a center of the secondary winding and connected to ground. Therefore, the secondary winding has two coils. When the transformer operates, during one cycle of the electric current, the leakage inductances in the first half cycle and in the later half cycle are caused respectively by the two coils of the secondary winding. Thus, the two sets of leakage inductances easily differ from each other. However, since the coils of the secondary winding can be wound together, distances respectively between the coils and the primary winding are able to be equal to each other and the leakage inductances in the coils of the secondary winding are also the same.
In another aspect, as for a conventional planar transformer, the secondary winding only has one turn, which is a single copper sheet or a copper foil layer formed on a circuit board. If the primary and secondary windings are disposed respectively on two opposite sides of the conventional transformer as in the abovementioned conventional structure, the distances between the two copper-sheeted secondary windings and the primary windings are not the same so the electric currents in the first and later half cycles are not balanced, either.
Moreover, in the conventional planar transformer, although the secondary winding has been replaced with the copper sheet or the copper foil layer on the circuit board, the primary winding of the conventional planar transformer is still made from a conducting wire. The conducting wire is wound in a disc-like form and then the disc-like shape is fixed by applying adhesive to, or a self-adhesive coating on the conducting wire. When the wound conducting wire is assembled into the transformer, it is hard to solder ends of the conducting wire to circuit of the circuit board, and safety distance between the ends of the conducting wire is also difficult to measure.
To overcome the shortcomings, the present invention provides a symmetric planar transformer having adjustable leakage inductance to mitigate or obviate the aforementioned problems.